Milky Way spins faster, has more mass than thought

  • Context: Undergrad 
  • Thread starter Thread starter Redbelly98
  • Start date Start date
  • Tags Tags
    Mass Milky way
Click For Summary

Discussion Overview

The discussion revolves around new estimates of the Milky Way's mass and the implications for the sun's revolution period around the galaxy. Participants explore the accuracy of these estimates and their impact on existing models of galactic dynamics.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested, Mathematical reasoning

Main Points Raised

  • Some participants reference a new estimate of the Milky Way's mass at 270 billion solar masses, questioning how this compares to previous estimates of 200-400 billion solar masses.
  • There is discussion about the sun's orbital period, with calculations suggesting it to be around 200 million years based on a radius of 28,000 light-years and an estimated speed of 600,000 mph.
  • One participant notes a discrepancy in earlier estimates of the sun's orbital period, recalling figures from the 1970s around 240 million years, and suggests that the new estimate may reflect a more precise measurement rather than a significant change in understanding.
  • Another participant provides an alternative calculation using a speed of 914,000 kph, resulting in an orbital period of approximately 208 million years, highlighting the variability in estimates based on different sources.
  • Concerns are raised about the accuracy of reported speeds in media articles, with a participant noting potential errors in significant figures during unit conversions.
  • There is interest in obtaining more technical reports that include error bars to better understand the reliability of the new mass estimates.

Areas of Agreement / Disagreement

Participants express varying views on the accuracy of the new mass estimates and their implications for the sun's orbital period. There is no consensus on the definitive value of the sun's revolution period, as different calculations yield slightly different results.

Contextual Notes

Participants acknowledge the limitations of current estimates, including the dependence on the methodology used to determine the mass and the assumptions made regarding the sun's orbit.

Who May Find This Useful

This discussion may be of interest to those studying astrophysics, galactic dynamics, or anyone following recent developments in astronomical research.

Redbelly98
Staff Emeritus
Science Advisor
Homework Helper
Insights Author
Messages
12,179
Reaction score
186
Saw this on Yahoo! News; the title pretty much sums it up.
http://news.yahoo.com/s/afp/20090105/ts_afp/usastronomy_090105234256

My question for the regular astro PFers: what would be the sun's revolution period around the galaxy based on this new information?
 
Last edited by a moderator:
Astronomy news on Phys.org
Redbelly98 said:
Saw this on Yahoo! News; the title pretty much sums it up.
http://news.yahoo.com/s/afp/20090105/ts_afp/usastronomy_090105234256

My question for the regular astro PFers: what would be the sun's revolution period around the galaxy based on this new information?

Redbelly, these results were under embargo until 5 Jan, yesterday. I searched yesterday and today for something more technical than the press-release material and couldn't find.

According to this report
http://www.guardian.co.uk/science/2009/jan/05/galaxy-collision-space-milky-way
the new estimate of the mass (including DM) is 270 billion solar mass. But this is similar to the estimates 200-400 billion that I have seen for some time!

So until I see some more technical report I don't know what to think. Maybe the real news is that they have narrowed down the uncertainty. And that the new more precise estimate is higher than yesterday's LOW estimates. However it turns out, we can address your question about the orbital period. The Harvard-Smithsonian press release is here
http://www.cfa.harvard.edu/press/2009/pr200903.html
and it implies the orbit radius is 28,000 LY, so assuming circular the circumf is 176,000 LY
and the estimated speed is 600,000 mph
So we just type into google something like
(176 000 lightyear) / (600 000 mph)
and google will say
(176 000 lightyear)/(600 000 mph) = 196 714 211 years
So we can quickly say the orbit period is 200 million years.

Indeed this is a tad shorter than what I remember from earlier. Back in 1970s I believe a common estimate was around 240 million years.
A good question would be if this 270 billion solar is right, then does that refer just to the mass inside the sun's orbit? Because their methodology seems to be good for finding the sun's orbital speed and that would just give a handle on what's inside our orbit. If they are trying to include an estimate of the outer part of the disc, then how are they doing that? Maybe someone else here will clarify.
 
Last edited by a moderator:
marcus said:
... The Harvard-Smithsonian press release is here
http://www.cfa.harvard.edu/press/2009/pr200903.html
and it implies the orbit radius is 28,000 LY, so assuming circular the circumf is 176,000 LY
and the estimated speed is 600,000 mph
Doh! That info is also in the Yahoo article I linked...somehow I had glazed over it before.

Indeed this is a tad shorter than what I remember from earlier. Back in 1970s I believe a common estimate was around 240 million years.
My notes have 250 Myr, based on google searches a year or 2 ago. I keep a list of time scales found in nature, from the Planck time on up to the age of the universe. The sun's orbital period is on my list, so when I read this article I realized I needed to update it.

Thanks marcus!

Mark
 
Thanks for starting the thread!
I found another estimate 914,000 kph (a.k.a. km/h)
(this time in a BBC article)
176000 lightyear/914000 kph
Google says:
"(176 000 lightyear) / (914 000 kph) = 207 821 117 years"
so 208 million years.
 
Thanks for starting the thread!
And thanks for joining it.

I'm interested in what numbers are given in the technical reports, with error bars. I imagine you'll see what those are eventually. The Yahoo article appears to make the all-too-common journalist error of too many sig figs after converting units:

500,000 mi/hr becomes 804,672 km/hr
600,000 mi/hr becomes 965,600 km/hr

It's likely the BBC's 914,000 kph is believable, since they probably didn't need to convert units.

And yes, I'm familiar with the Google calculator. It even recognizes "pi", so it can do the entire calculation:

(2*pi*28 lightyear) / (914000 kph) = 207 737.503 years
 
They talked about this on NPR radio today:
http://www.sciencefriday.com/program/archives/200901093
Click the play bar under the word "Listen" to the left. I haven't heard it myself yet.
 
Last edited by a moderator:

Similar threads

Graduate Possible Intermediate-Mass Black Hole in Milky Way Galaxy
  • · Replies 1 ·
Replies
1
Views
2K
Undergrad Do stars have any lower mass limit?
  • · Replies 8 ·
Replies
8
Views
3K
High School Neutron star 4U 1820-30 spins at 716 revolutions per second
  • · Replies 0 ·
Replies
0
Views
3K
Graduate Can I Travel Faster Than Light While Considering Relativity?
  • · Replies 6 ·
Replies
6
Views
16K
How Is Stellar Mass Distributed in the Milky Way?
  • · Replies 1 ·
Replies
1
Views
3K
Undergrad What Key Issues Should Cosmologists Address in the ΛCDM Model?
  • · Replies 1 ·
Replies
1
Views
3K
Estimating the total mass of the galaxy (using Kepler's Thid Law)
  • · Replies 4 ·
Replies
4
Views
28K
Mathematica This Week's Finds in Mathematical Physics (Week 257)
  • · Replies 5 ·
Replies
5
Views
5K